U.S. patent application number 14/364971 was filed with the patent office on 2014-11-27 for method for transmitting at least one multi-carrier signal consisting of ofdm-oqam symbols.
The applicant listed for this patent is ORANGE. Invention is credited to Thierry Dubois, Maryfine Helard, Bruno Jahan, Dinh Thuy Phan Huy, Pierre Siohan.
Application Number | 20140348252 14/364971 |
Document ID | / |
Family ID | 45809227 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140348252 |
Kind Code |
A1 |
Siohan; Pierre ; et
al. |
November 27, 2014 |
METHOD FOR TRANSMITTING AT LEAST ONE MULTI-CARRIER SIGNAL
CONSISTING OF OFDM-OQAM SYMBOLS
Abstract
Disclosed is a transmission method of transmitting at least one
multicarrier signal made up of OFDM-OQAM symbols to a receive
antenna of a receiver device, the transmission method being for use
by a transmitter device having at least one transmit antenna. In
one embodiment, the method comprises, for each transmit antenna of
the transmitter device, filtering OFDM-OQAM symbols associated with
the transmit antenna using a time-reversal prefilter defined from
an estimate of the transmission channel between the transmit
antenna and the receive antenna and transmitting the filtered
OFDM-OQAM symbols via the transmit antenna over the transmission
channel.
Inventors: |
Siohan; Pierre; (Rennes,
FR) ; Jahan; Bruno; (Tinteniac, FR) ; Phan
Huy; Dinh Thuy; (Paris, FR) ; Helard; Maryfine;
(Rennes, FR) ; Dubois; Thierry; (Rennes,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ORANGE |
Paris |
|
FR |
|
|
Family ID: |
45809227 |
Appl. No.: |
14/364971 |
Filed: |
December 19, 2012 |
PCT Filed: |
December 19, 2012 |
PCT NO: |
PCT/FR2012/052995 |
371 Date: |
June 12, 2014 |
Current U.S.
Class: |
375/261 |
Current CPC
Class: |
H04L 27/264 20130101;
H04L 27/2698 20130101; H04L 27/2626 20130101; H04L 1/0668 20130101;
H04L 27/368 20130101; H04B 7/0669 20130101 |
Class at
Publication: |
375/261 |
International
Class: |
H04L 27/26 20060101
H04L027/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2011 |
FR |
1162407 |
Claims
1. A transmission method for transmitting at least one multicarrier
signal made up of OFDM-OQAM symbols to a receive antenna of a
receiver device, said transmission method being for use by a
transmitter device having at least one transmit antenna, said
transmission method comprising, for each transmit antenna of the
transmitter device: filtering OFDM-OQAM symbols associated with the
transmit antenna using a time-reversal prefilter defined from an
estimate of the transmission channel between said transmit antenna
and said receive antenna; and transmitting the filtered OFDM-OQAM
symbols via said transmit antenna over said transmission
channel.
2. A transmission method according to claim 1, wherein the
transmitter device has a number N of transmit antennas greater than
1, said transmission method further comprising a coding process
during which coding is applied to real value symbols, the coding
being defined by a determined real coding matrix generating N coded
symbol sequences associated with said N transmit antennas, a coded
symbol of a coded symbol sequence associated with a transmit
antenna modulating a carrier of a said OFDM-OQAM symbol associated
with that antenna.
3. A transmission method according to claim 2, wherein the real
coding matrix is a real orthogonal matrix.
4. A transmission method according to claim 2, wherein the coding
applied during the coding step has a coding rate of 1.
5. A transmission method according to claim 2, wherein the coding
applied during the coding step is space-time coding.
6. A transmission method according to claim 2, wherein the coding
applied during the coding step is space-frequency coding.
7. A computer program including instructions for executing steps of
the transmission method according to claim 1 when said program is
executed by a computer.
8. A non-transitory computer readable storage medium storing a
computer program including instructions for executing steps of the
transmission method according to claim 1.
9. A reception method for receiving an OFDM-OQAM type multicarrier
signal, referred to as a received multicarrier signal, which method
is performed by a receiver device having a receive antenna, said
received multicarrier signal resulting from at least one OFDM-OQAM
type multicarrier signal being transmitted over a propagation
channel in accordance with a transmission method according to claim
1 by a transmitter device having at least one transmit antenna,
said reception method including a performing OFDM-OQAM demodulation
of said received multicarrier signal.
10. A communication method of transmitting at least one OFDM-OFQAM
type multicarrier signal performed by a communications system
comprising a transmitter device having at least one transmit
antenna and a receiver device having a receive antenna, said method
comprising: said transmitter device transmitting said at least one
multicarrier signal in accordance with a transmission method
according to claim 1; and said receiver device receiving said at
least one multicarrier signal in accordance with a reception method
according to claim 9.
11. A transmitter device having at least one transmit antenna and
suitable for transmitting at least one OFDM-OQAM type multicarrier
signal via said at least one transmit antenna to a receive antenna
of a receiver device, said transmitter device comprising in
association with each transmit antenna: a component configured to
filter OFDM-OQAM symbols associated with that transmit antenna,
said component comprising a time-reversal prefilter defined on the
basis of an estimate of a transmission channel between said
transmit antenna and said receive antenna; and a component
configured to transmit the filtered OFDM-OQAM symbols via said
transmit antenna over said transmission channel.
12. A transmitter device according to claim 11, having a number N
of transmit antennas greater than 1 and further comprising a
component configured to apply coding to real value symbols, the
coding being defined by a determined real coding matrix generating
N coded symbol sequences associated with said N transmit antennas,
a coded symbol of a coded symbol sequence associated with a
transmit antenna modulating a carrier of a said OFDM-OQAM symbol
associated with that antenna.
13. A receiver device suitable for receiving an OFDM-OQAM type
multicarrier signal, referred to as a received multicarrier signal,
the receiver device having a receive antenna, said received
multicarrier signal being the result of at least one OFDM-OQAM type
multicarrier signal being transmitted over a propagation channel by
a transmitter device having at least one transmit antenna and in
accordance with claim 11, said receiver device including an
OFDM-OQAM demodulator suitable for demodulating received
multicarrier signals.
14. A communications system comprising: a transmitter device in
accordance with claim 11 having at least one transmit antenna
suitable for transmitting at least one OFDM-OQAM type multicarrier
signal over said at least one transmit antenna; and a receiver
device in accordance with claim 13, including a receive antenna and
suitable for receiving said at least one multicarrier signal over
said receive antenna.
15. A signal made up of at least one OFDM-OQAM type multicarrier
signal transmitted by a transmitter device having at least one
transmit antenna in accordance with a transmission method according
to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to the general field of
telecommunications.
[0002] The invention relates more particularly to transmitting
digital signals based on multicarrier modulation of the orthogonal
frequency division multiplex-offset quadrature amplitude modulation
(OFDM-OQAM) type in the context of systems having a number of
transmit antennas N greater than or equal to 1 and a receive
antenna. Such systems are also known as single-input single-output
(SISO) systems in the presence of a single transmit antenna and a
single receive antenna, or as multiple-input single-output (MISO)
systems in the presence of a plurality of transmit antennas and a
single receive antenna.
[0003] The invention can be applied to the field of wired
communications (e.g. x digital subscriber line (xDSL), power line
communication (PLC), optical fiber, etc.), or wireless
communications (e.g. beyond third generation (B3G) systems,
wireless local area network (WLAN) systems, etc.), for uplinks
(terminal to an access point/base station) and/or for downlinks
(access point/base station to terminal).
[0004] In known manner, the noise associated with the imperfections
of communications systems and with the physical nature of the
components used in such systems (such as the antennas, for example)
affects the transmission of digital signals. Such signals are also
subjected to deformation when they propagate between the transmit
antenna (s) and the receive antenna (e.g. via an air channel). In
the description below, the concept of a transmission or propagation
channel between a transmit antenna and a receive antenna is used to
cover not only the effects of the medium via which the digital
signal propagates between the transmit antenna and the receive
antenna (e.g. wireless channel or wired channel), but also the
effects induced by the transmit and receive antennas on the digital
signal.
[0005] The invention has a preferred but non-limiting application
in the field of data transmission over so-called frequency
selective transmission channels (multipath channels) with
variations over time that are relatively slow.
[0006] In known manner, the frequency selectivity of a transmission
channel is associated with the digital signal that is it desired to
transmit over the channel: it is representative of the fact that
the digital signal has frequency components that are attenuated
differently by the transmission channel. In other words, this
phenomenon appears when the bandwidth of the signal that is to be
transmitted is much greater than the coherence bandwidth of the
transmission channel, where the coherence bandwidth of a channel is
defined as the minimum bandwidth at which two attenuations of the
channels are independent. Compensating for the effects of the
distortions introduced by multipath channels is then performed with
the help of equalization techniques.
[0007] In this context, it is generally accepted that multicarrier
transmission systems (or multicarrier modulation (MCM) systems),
such as OFDM systems in particular, present numerous advantages. By
transmitting data over a plurality of carrier frequencies in
parallel (i.e. frequency multiplexing on a plurality of carrier
frequencies also referred to as carriers or subcarriers),
multicarrier transmission systems take the information that is for
transmission at a high data rate and spread it over a large number
of individual subbands that are modulated at low rates. This
advantageously makes it possible to replace the step of equalizing
a multipath channel, which may turn out to be complex in the time
domain for single-carrier systems, with a step of performing
equalization that is simple (having one coefficient per carrier)
and that is performed in the frequency domain.
[0008] Orthogonal frequency division multiplexing (OFDM) is a
multicarrier modulation technique that imposes a constraint of
orthogonality between the subcarriers in order to limit
interference between carriers (known as intercarrier interference
(ICI)), while optimizing spectrum occupation. OFDM also benefits
from implementation schemes that are simple and effective on the
basis of inverse fast Fourier transforms (IFFTs) on transmission
and of fast Fourier transforms (FFTs) on reception.
[0009] The constraint of orthogonality between the subcarriers is
ensured by using a rectangular function (.PI. gate function) for
shaping the multicarrier channel. Adding redundancy in the form of
a cyclic prefix (CP) or of a guard interval (e.g. made of zeros) at
the output from the inverse fast Fourier transform also makes it
possible to limit the distortion due to the interference introduced
by the channel between symbols (known as intersymbol interference)
(ISI)), and been carriers.
[0010] An OFDM signal s.sub.CP-OFDM[k] with a cyclic prefix (a
CP-OFDM signal) in baseband, with discrete time, and with M
subcarriers at an instant kTe, where Te designates the sampling
period, is expressed by the following equation:
s CP - OFDM [ k ] = m = 0 M - 1 n = - .infin. + .infin. c m , n
.PI. [ k - n ( M + L CP ) ] j 2 .pi. M mk ##EQU00001##
where j.sup.2=-1, .PI. designates a gate function of duration M,
c.sub.m,n is a complex symbol (e.g. a symbol obtained by quadrature
amplitude modulation (QAM)) transmitted over the carrier m at
instant n, and L.sub.CP is the length of the cyclic prefix in
number of samples.
[0011] Nevertheless, although adding a cyclic prefix (or a guard
interval) of length L.sub.CP longer than the greatest length of the
channel makes channel equalization easier by avoiding problems of
ISI, it gives rise to a loss of spectrum efficiency that increases
with increasing length of the cyclic prefix (or guard interval).
The cyclic prefix or guard interval that is added does not convey
any useful information in order to guarantee that the information
that is received and processed on reception comes from a single
multicarrier symbol.
[0012] In order to mitigate that drawback, it is known to make use
of modulation of the orthogonal frequency division
multiplexing/offset quadrature amplitude modulation (OFDM/OQAM)
type. On each subcarrier, this modulation makes use of a complex
QAM symbol c.sub.m,n being resolved (decomposed) into a pair of
real symbols constituted by the real part (c.sub.m,n) and the
imaginary part I(c.sub.m,n) of the complex symbol c.sub.m,n applied
to two half-symbol times. The real and imaginary parts of the
complex symbols that are to be transmitted are also offset by one
half-symbol time between two successive subcarriers. This
decomposition into real symbols advantageously makes it possible to
relax the constraint of orthogonality between the subcarriers to
the domain of reals, thereby facilitating the design of orthogonal
functions for shaping the multicarrier signal (also known as
prototype functions or filters) that are thoroughly localized in
terms of frequency and time.
[0013] The OFDM/OQAM signal s.sub.OFDM-OQAM[k] in baseband and in
discrete time for M subcarriers at the instant kTe, where Te
designates the sampling period, can thus be expressed in the
following form:
s OFDM - OQAM [ k ] = m = 0 M - 1 n = - .infin. + .infin. a m , n f
[ k - n M 2 ] j 2 .pi. M m ( k - ( LF - 1 ) / 2 ) j .phi. m , n f m
, n [ k ] ##EQU00002##
where the coefficients a.sub.m,n are real coefficients (e.g. pulse
amplitude modulation (PAM) symbols), f[ ] designates a prototype
filter of length LF, and .phi..sub.m,n designates a phase term,
e.g. selected to be equal to
.pi. 2 ( m + n ) . ##EQU00003##
[0014] Thus, the OFDM/OQAM modulation is freed from the presence of
a guard interval or a cyclic prefix by a suitable selection of the
prototype filter f modulating each subcarrier of the signal in such
a manner as to ensure that each of these subcarriers is well
localized in time and in frequency, and satisfying a real
orthogonality constraint between the subcarriers that is expressed
as follows:
{ f m , n , f m ' , n ' } = { k = - .infin. + .infin. f m , n [ k ]
f m ' , n ' * [ k ] } = .delta. m , m ' .delta. n , n '
##EQU00004##
where <g,h> designates the scalar product between g and h.
The scalar product <f.sub.m,n, f.sub.m',n'> is thus a pure
imaginary number for (m,n).noteq.(m',n'). For simplification
purposes in the description below, the following notation is
used:
f.sub.m,n.sup.p,q=--jf.sub.m,n,f.sub.p,q
[0015] By way of example, a known prototype filter that satisfies
this constraint is the prototype filter obtained from the IOTA
function as described in patent application FR 2 733 869, or the
TFL1 prototype filter used in the document by C. Lele et al.
entitled "Channel estimation methods for preamble-based OFDM/OQAM
modulations", European Wireless, April 2007 (given reference D1 in
the description below).
[0016] Nevertheless, in spite of using a prototype filter that is
well localized in time and in frequency, OFDM/OQAM modulation, by
construction, produces an imaginary intrinsic interference term.
Using conventional assumptions concerning the transmission model
(channel invariant in a neighborhood .OMEGA..sub..DELTA.m,.DELTA.n
departing by no more than .+-..DELTA.m, .+-..DELTA.n around each
time-frequency point of coordinates (m,n), prototype filter f well
localized in time and in frequency and shifted from the prototype
filter that are invariant for a maximum delay of the channel equal
to a determined number of samples), it is easy to show that for a
SISO system, after transmission over a frequency selective channel
with disturbance by additive noise written .eta., the demodulation
signal can be written in the following form:
y.sub.m,n=h.sub.m,n(a.sub.m,n+ja.sub.m,n.sup.(i))+J.sub.m,n+.eta..sub.m,-
n
where: [0017] h.sub.m,n designates the value of the complex channel
on the subcarrier m at instant n; [0018] .eta..sub.m,n designates
the noise component at instant n on the subcarrier m; [0019]
ja.sub.m,n.sup.(i) designates a purely imaginary intrinsic
interference term affecting the symbol a.sub.m,n and depending on
its neighboring symbols at the instant n as given by:
[0019] a m , n ( i ) = ( p , q ) .epsilon..OMEGA. .DELTA. m ,
.DELTA. n - ( 0 , 0 ) a m + p , n + p f p , q m , n
##EQU00005##
with f.sub.m,n.sup.p,q=-jf.sub.m,n,f.sub.p,q; and [0020] J.sub.m,n
designates an interference term created by symbols situated outside
the neighborhood of the symbol a.sub.m,n.
[0021] Calculation of the interference terms is described in
greater detail in document D1 and is not repeated herein.
[0022] If firstly the prototype filter f is well localized in
frequency and in time, and if secondly the channel is not
exclusively frequency selective and/or the signal-to-noise ratio is
not too great, the term J.sub.m,n can be ignored compared with the
noise term .eta..sub.m,n. It should be observed that this
approximation is appropriate in a large number of scenarios that
are to be encountered in practice. The demodulated signal can then
be approximated as follows:
y.sub.m,n.apprxeq.h.sub.m,n(a.sub.m,n+ja.sub.m,n.sup.(i))+.eta..sub.m,n
[0023] Starting from this approximation, various techniques can
then be envisaged on transmission and on reception for eliminating
the intrinsic interference term a.sub.m,n.sup.(i) for a SISO
system. By way of example, one such technique, mentioned in
document D1, consists in using a ZF equalizer on reception having
one coefficient per carrier applied to the real part of the
demodulated signal y.sub.m,n.
[0024] Nevertheless, although those techniques are effective for a
SISO system (i.e. a system with a single transmit antenna and a
single receive antenna), they are not easily transposable to a
multiantenna system of the MISO type, and in particular to a system
making use of space-time coding, such as for example the orthogonal
coding scheme of coding rate 1 defined for two transmit antennas in
the document by S. Alamouti entitled "A simple transmit diversity
technique for wireless communications", IEEE Journal of Selected
Areas Communication, 1988, No. 16, pp. 1451-1458.
[0025] The space-time coding scheme produced by Alamouti
corresponds to an open loop coding system in which two successive
complex symbols s1 and s2 are transmitted over two transmit
antennas in compliance with the following code matrix:
G 2 = [ s 1 - s 2 * s 2 s 1 * ] antennas .dwnarw. , time .fwdarw.
##EQU00006##
where s* designates the complex conjugate of the symbol s. The rows
of the matrix G2 give the symbols transmitted over the various
transmit antennas: thus, the symbols s1 and then -s2* are
transmitted over the first antenna, while the symbols s2 and s1*
are transmitted over the second antenna.
[0026] The coding matrix G2 is a complex orthogonal matrix, i.e.
G2G2.sup.H=I, where I designates the identity matrix of dimensions
2.times.2 and .sup.H designates the Hermetian operator. Thus, the
coding scheme proposed by Alamouti advantageously offers a coding
rate of 1 while ensuring on reception that the symbols transmitted
over each antenna are decoupled, thus making it possible to use
simple linear decoding with maximum likelihood.
[0027] Applying the Alamouti coding scheme to OFDM modulation leads
to the coding matrix GC2 being rewritten, e.g. in the following
form:
GC 2 = [ c m , n - c m , n + 1 * c m , n + 1 c m , n * ]
##EQU00007##
where c.sub.m,n designates the complex symbol transmitted at an
instant n on a subcarrier m.
[0028] A similar coding scheme can also be defined for modulation
of OFDM-OQAM type, taking account of the fact that, as mentioned
above, OFDM-OQAM modulation resolves each complex symbol c.sub.m,n
into a pair of real symbols ((c.sub.m,n) and I(c.sub.m,n)) that are
spaced apart on the same subcarrier by a half symbol time T/2
(where T designates the duration of a complex symbol), and that are
also offset by a half-symbol time between two consecutive
subcarriers. Using the notation a.sub.m,2n+k,i for the real symbols
transmitted over the carrier m at four successive instants
(2n+k)T/2, k=0, . . . , 3, over antenna i, i=0,1, it is possible to
define the following orthogonal coding scheme:
a.sub.m,2n,0=(c.sub.m,2n)
a.sub.m,2n,1=(c.sub.m,2n+1)
a.sub.m,2n+1,0=I(c.sub.m,2n)
a.sub.m,2n+1,1=I(c.sub.m,2n+1)
a.sub.m,2n+2,0=((c.sub.m,2n+1)*)=-(c.sub.m,2n+1)=a.sub.m,2n,1
a.sub.m,2n+2,1=((c.sub.m,2n)*)=(c.sub.m,2n)=a.sub.m,2n,0
a.sub.m,2n+3,0=I((c.sub.m,2n+1)*)=I(c.sub.m,2n+1)=a.sub.m,2n+1,1
a.sub.m,2n+3,1=I(c.sub.m,2n))=-I(c.sub.m,2n)=a.sub.m,2n+1,0
[0029] In similar manner, for SISO, if h.sub.m,n,i designates the
gainl of the complex channel between the transmit antenna i and the
receive antenna for the subcarrier m at instant nT/2, and if it is
assumed that this gain is constant between the instants 2nT/2 and
(2n+3)T/2, then the signal received on the receive antenna for the
subcarrier m is given by:
y.sub.m,2n=h.sub.m,2n,0(a.sub.m,2n,0+ja.sub.m,2n,0.sup.(i))+h.sub.m,2n,1-
(a.sub.m,2n,1+ja.sub.m,2n,1.sup.(i))+n.sub.m,2n,0,
y.sub.m,2n+1=h.sub.m,2n,0(a.sub.m,2n+1,0+ja.sub.m,2n+1,0.sup.(i))+h.sub.-
m,2n,1(a.sub.m,2n+1,1+ja.sub.m,2n+1,1.sup.(i))+n.sub.m,2n+1,1,
y.sub.m,2n+2=h.sub.m,2n,0(a.sub.m,2n+2,0+ja.sub.m,2n+2,0.sup.(i))+h.sub.-
m,2n,1(a.sub.m,2n+2,1+ja.sub.m,2n+2,1.sup.(i))+n.sub.m,2n+2,0,
y.sub.m,2n+3=h.sub.m,2n,0(a.sub.m,2n+3,0+ja.sub.m,2n+3,0.sup.(i))+h.sub.-
m,2n,1(a.sub.m,2n+3,1+ja.sub.m,2n+3,1.sup.(i))+n.sub.m,2n+3,1,
where a.sub.m,n,i.sup.(i) designates the intrinsic interference
affecting the real symbol a.sub.m,n,i depending on its neighboring
symbols at instant n.
[0030] By writing:
z.sub.m,2n=y.sub.m,2n+jy.sub.m,2n+1 and
z.sub.m,2n+1=y.sub.m,2n+2+jy.sub.m,2n+3
the following expression can be obtained from the above equations
after performing a few calculations that are described in greater
detail in document D1:
[ z m , 2 n ( z m , 2 n + 1 ) * ] z 2 n _ = [ h m , 2 n , 0 h m , 2
n , 1 h m , 2 n , 1 * - h m , 2 n , 0 * ] Q 2 n _ [ c m , 2 n c m ,
2 n + 1 ] c 2 n _ + [ h m , 2 n , 0 h m , 2 n , 1 0 0 0 0 h m , 2 n
, 1 * - h m , 2 n , 0 * ] K 2 n _ [ x m , 2 n , 0 x m , 2 n , 1 x m
, 2 n + 2 , 0 x m , 2 n + 2 , 0 ] x 2 n _ + [ .mu. m , 2 n .mu. m ,
2 n + 1 * ] .mu. 2 n _ ##EQU00008##
where: [0031] the matrix Q.sub.2n is an orthogonal matrix; [0032]
.mu..sub.2n is a noise component; and [0033] x.sub.2n is a vector
with components that are linear combinations of intrinsic
interference terms.
[0034] This expression shows that unlike the SISO situation, in a
MISO system using Alamouti type coding, the signal received on the
receive antenna presents an intrinsic interference term
K.sub.2nx.sub.2n that, even in the absence of noise, is difficult
to eliminate.
[0035] Solutions exist in the represent state of the art for
remedying this drawback. Nevertheless, most of them require either
complex reception schemes to be implemented, or else the addition
of a cyclic prefix, thereby giving rise to a loss of spectrum
efficiency.
[0036] There therefore exists a need for an OFDM-OQAM type
transmission scheme that does not lead to a loss of spectrum
efficiency and that can be used in the context of a SISO system or
a MISO system without requiring complex reception algorithms to be
performed.
OBJECT AND SUMMARY OF THE INVENTION
[0037] The invention satisfies this need in particular by proposing
a transmission method for transmitting at least one OFDM-OQAM type
multicarrier signal to a receive antenna of a receiver device, the
method being for performing by a transmitter device having at least
one transmit antenna, the method comprising, for each transmit
antenna of the transmitter device: [0038] a step of filtering
OFDM-OQAM symbols associated with the transmit antenna by means of
a time-reversal prefilter defined from an estimate of the
transmission channel between the transmit antenna and the receive
antenna; and [0039] a step of transmitting the filtered OFDM-OQAM
symbols via the transmit antenna over the transmission channel.
[0040] More precisely, if the transmission channel between the
transmit antenna under consideration and the receive antenna is
modeled by a linear filter having a finite impulse response h(t) (a
conventional assumption for modeling a multicarrier channel), and
if h (t) designates an estimate of the response of the transmission
channel, then the time-reversal prefilter applied during the
filtering step is a matched filter having the impulse response
h*(-t). In equivalent manner, if H(z) designates the z transform of
the discrete version of the transmission channel, and H(z)
designates the z transform of the estimate of the transmission
channel, then the time-reversal prefilter applied during the
filtering step has its z transform given by H*(z.sup.-1).
[0041] Correspondingly, the invention also provides a transmitter
device having at least one transmit antenna and suitable for
transmitting at least one OFDM-OQAM type multicarrier signal to a
receive antenna of a receiver device, the transmitter device
comprising in association with each transmit antenna: [0042] means
for filtering OFDM-OQAM symbols associated with that transmit
antenna, the means comprising a time-reversal prefilter defined on
the basis of an estimate of a transmission channel between the
transmit antenna and the receive antenna; and [0043] means for
transmitting the filtered OFDM-OQAM symbols via the transmit
antenna over the transmission channel.
[0044] The invention thus provides a novel scheme for transmitting
an OFDM-OQAM type multicarrier signal operating in a closed loop
and using an estimate of the transmission channel between the
transmit antenna(s) and the receive antenna. The estimation may be
performed for example at the receiver device and returned to the
transmitter device via a return channel.
[0045] More precisely, the invention proposes transmitting
OFDM-OQAM symbols associated with a transmit antenna of the
transmitter device over an equivalent channel resulting from
convolution of the time-reversal prefilter with the transmission
channel corresponding to said transmit antenna.
[0046] Assuming that the estimate of the channel at the transmitter
device is perfect, it can easily be shown that the equivalent
channel is a symmetrical conjugate channel with a frequency
response that is real. This has the consequence that the real
orthogonality constraint on the subcarriers that is satisfied by
the prototype filter advantageously continues to be satisfied after
passing via the equivalent channel, since its frequency response is
real, even if the transmission channel associated with each antenna
is complex.
[0047] This avoids generating a purely imaginary intrinsic
interference term that is difficult to eliminate, such as the term
generated for prior art OFDM-OQAM modulation. The invention
therefore does not require, a fortiori, the use of reception
schemes that are complex in order to eliminate this interference,
and on the contrary it is possible to make use of conventional
linear reception schemes.
[0048] It should be observed that the equivalent channel has a
spread of delays that is greater than that of the complex
transmission channel h(t). Nevertheless, this delay does not harm
the performance of the OFDM-OQAM modulation since the power profile
of the equivalent channel is also more "concentrated" around its
central coefficient in the sense that the central coefficient has a
larger real amplitude (equal to the sum of the amplitudes squared
of each path) and the coefficients associated with the multiple
paths are attenuated more strongly than the transmission channel
between the transmit antenna under consideration and the receive
antenna. In addition, the solution proposed by the invention does
not require a cyclic prefix or a guard interval to be added, so
this spreading of delays has no impact properly speaking on the
spectrum efficiency of the transmission scheme.
[0049] The transmission scheme proposed by the invention can
advantageously be used in the context of a SISO or a MISO
communications system, and in particular a MISO communications
system using space-time or space-frequency type coding.
[0050] Thus, in a particular implementation in which the
transmitter device has a number N of transmit antennas greater than
1, the transmission method of the invention further comprises a
coding step during which coding is applied to real value symbols,
the coding being defined by a determined real coding matrix
generating N coded symbol sequences associated with said N transmit
antennas, a coded symbol of a coded symbol sequence associated with
a transmit antenna modulating a carrier of the OFDM-OQAM symbol
associated with that antenna.
[0051] Preferably, the coding matrix is a real orthogonal matrix.
Nevertheless, the invention also applies to a non-orthogonal coding
matrix.
[0052] By way of example, the coding applied during the coding step
may be space-time coding enabling the real value symbols to be
coded in a space dimension and in a time dimension (i.e. over a
plurality of transmit antennas and distributed over a plurality of
symbol times).
[0053] In a variant, the coding applied during the coding step is
space-frequency coding serving to code the real value symbols in a
space dimension and in a frequency dimension (i.e. over a plurality
of transmit antennas and over a plurality of OFDM modulation
subcarriers).
[0054] Preferably, the coding applied has a coding rate of 1, so as
to maximize spectrum efficiency. Nevertheless, the invention also
applies to coding at lower rates.
[0055] In other words, by combining OFDM-OQAM modulation and the
principle of time reversal, the invention advantageously makes it
possible to use real space-time or space-frequency codes, and to do
so without loss of information since the equivalent channel is a
real channel. The use of an orthogonal space-time or
space-frequency code in the transmission scheme also makes it
possible to benefit from the diversity made available by the
presence of multiple transmit antennas, while also ensuring that
the multicarrier signal resulting from the combination of the
signals received from these multiple antennas is simple to
demodulate.
[0056] Real orthogonal space-time (or space-frequency) codes are
known to exist that have the maximum code rate (i.e. a rate of 1),
regardless of the number of transmit antennas under consideration.
In contrast, complex orthogonal space-time (or space-frequency)
codes having a coding rate of 1 exist only when the number of
transmit antennas is equal to two.
[0057] The invention thus makes it possible to use an orthogonal
space-time or space-frequency code of maximum coding rate
regardless of the number of transmit antennas of the transmitter
device.
[0058] It should be observed that the combination of an orthogonal
real space-time or space-frequency code with OFDM-OQAM modulation
and the principle of time reversal as proposed by the invention is
not the result of an operation that is obvious to the person
skilled in the art. It assumes very accurate knowledge and astute
consideration of the conditions that are required for orthogonality
and that are associated with three distinct domains associated
respectively with OFDM-OQAM modulation, with
space-time/space-frequency coding, and with time reversal.
[0059] In a particular implementation, the various steps of the
transmission method are determined by computer program
instructions.
[0060] Consequently, the invention also provides a computer program
on a data medium, the program being suitable for being implemented
in a transmitter device or more generally in a computer, the
program including instructions adapted to performing steps of a
transmission method as described above.
[0061] The program may use any programming language, and be in the
form of source code, object code, or code intermediate between
source code and object code, such as in a partially compiled form,
or in any other desirable form.
[0062] The invention also provides a computer readable data medium
including instructions of a computer program as specified
above.
[0063] The data medium may be any entity or device capable of
storing the program. For example, the medium may comprise storage
means, such as a read only memory (ROM), e.g. a compact disk (CD)
ROM, or a microelectronic circuit ROM, or indeed magnetic recording
means, e.g. a floppy disk or a hard disk.
[0064] Furthermore, the data medium may be a transmissible medium
such as an electrical or optical signal, that may be conveyed via
an electrical or optical cable, by radio, or by other means. The
program of the invention may in particular be downloaded from an
Internet type network.
[0065] Alternatively, the data medium may be an integrated circuit
in which the program is incorporated, the circuit being adapted to
execute or to be used in the execution of the method in
question.
[0066] In another aspect, the invention also provides a reception
method for receiving an OFDM-OQAM type multicarrier signal,
referred to as a received multicarrier signal, which method is
performed by a receiver device having a receive antenna, the
received multicarrier signal resulting from at least one OFDM-OQAM
type multicarrier signal being transmitted over a propagation
channel in accordance with a transmission method of the invention
by means of a transmitter device having at least one transmit
antenna, the reception method including a step of performing
OFDM-OQAM demodulation of the received multicarrier signal.
[0067] Correspondingly, the invention also provides a receiver
device suitable for receiving an OFDM-OQAM type multicarrier
signal, referred to as a received multicarrier signal, the receiver
device having a receive antenna, the received multicarrier signal
being the result of at least one OFDM-OQAM type multicarrier signal
being transmitted over a propagation channel by a transmitter
device having at least one transmit antenna and in accordance with
the invention, the receiver device including an OFDM-OQAM
demodulator suitable for demodulating received multicarrier
signals.
[0068] In another aspect, the invention also provides a
communication method of transmitting at least one OFDM-OQAM type
multicarrier signal performed by a communications system comprising
a transmitter device having at least one transmit antenna and a
receiver device having a receive antenna, the method comprising:
[0069] said transmitter device transmitting said at least one
multicarrier signal in accordance with the transmission method of
the invention; and [0070] the receiver device receiving said at
least one multicarrier signal via its receive antenna in accordance
with the reception method of the invention.
[0071] Correspondingly, the invention also provides a
communications system comprising: [0072] a transmitter device in
accordance with the invention having at least one transmit antenna
and suitable for transmitting at least one multicarrier signal over
said at least one transmit antenna; and [0073] a receiver device
according to the invention including a receive antenna and suitable
for receiving said at least one multicarrier signal over said
receive antenna.
[0074] It should be observed that if the transmitter device has a
plurality of transmit antennas, a plurality of multicarrier signals
are transmitted by these transmit antennas respectively such that
the receiver device receives a combination of these multicarrier
signals via its receive antenna, and demodulates this combination
of signals.
[0075] In another aspect, the invention also provides a signal made
up of at least one OFDM-OQAM type multicarrier signal transmitted
by a transmitter device including at least one transmit antenna in
accordance with the transmission method of the invention.
[0076] The communication method, the communications system, and the
signal of the invention have the same advantages as those mentioned
above for the transmission method and the transmitter device of the
invention.
[0077] In other implementations and embodiments, it is also
possible to envisage that the transmission method, the
communication method, the transmitter device, and the
communications system of the invention present in combination all
or some of the above-specified characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] Other characteristics and advantages of the present
invention appear from the following description made with reference
to the accompanying drawings that show an embodiment having no
limiting character. In the figures:
[0079] FIG. 1 is a diagram of the new transmission scheme proposed
by the invention;
[0080] FIG. 2 shows a transmitter device, a receiver device, and a
SISO communications system in accordance with the invention in a
first embodiment;
[0081] FIG. 3 shows the main steps of a transmission method, a
reception method, and a communication method in accordance with the
invention in a first implementation in which they are performed
respectively by the transmitter device, by the receiver device, and
by the SISO communications system of FIG. 2;
[0082] FIG. 4 shows a transmitter device, a receiver device, and a
MISO communications system in accordance with the invention in a
second embodiment; and
[0083] FIG. 5 shows the main steps of a transmission method, a
reception method, and a communication method in accordance with the
invention in a second implementation in which they are performed
respectively by the transmitter device, by the receiver device, and
by the MISO communications system of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0084] As mentioned above, the invention advantageously proposes a
novel multicarrier signal transmission scheme of the OFDM-OQAM type
suitable for use in a SISO or MISO system and operating in a closed
loop on the basis of knowledge about the transmission channel (or
propagation channel). This new scheme is shown in FIG. 1.
[0085] The notion of a transmission channel between a transmit
antenna and a receive antenna in the meaning of the invention
includes the effects of the propagation channel proper between the
transmit antenna and the receive antenna, and also the
contributions of the transmit antenna and of the receive antenna.
It should be observed that in a multiple-input and single-output
system (i.e. having a plurality of transmit antennas and a single
receive antenna), there are as many transmission channels as there
are (transmit antenna, receive antenna) pairs, or in other words as
many as there are transmit antennas.
[0086] More precisely, the general principle of the invention
relies on applying a technique of time reversing the transmission
channel (between each transmit antenna and the receive antenna) to
multicarrier modulation of the OFDM-OQAM type.
[0087] In other words, and with reference to FIG. 1, the OFDM-OQAM
symbols are convoluted prior to being transmitted via the transmit
antenna over the transmission channel by a time-reversal prefilter
(i.e. a matched filter) of coefficients that are obtained by time
reversal of the coefficients of the transmission channel (or more
precisely of an estimate of the transmission channel).
[0088] It is assumed that the transmission channel is a
frequency-selective channel made up of L paths, and modeled by a
linear filter having complex coefficients h.sub.0, h.sub.1, . . . ,
h.sub.L-1. The vector of complex coefficients of the channel is
written h=(h.sub.0, h.sub.1, . . . , h.sub.L-1). In the description
below, the transmission channel is designated and specified either
by the channel coefficients, or by its time impulse response, or by
its z transform, these various representations being equivalent.
The same applies for the time-reversal prefilter.
[0089] The coefficients h.sub.l, l=0, . . . , L-1 are complex
coefficients in which the imaginary part and the real part are
distributed in compliance with a centered normal distribution, and
the norm .parallel.h.sub.l.parallel. follows a Rayleigh
distribution. The z transform, written H(z), of the impulse
response h(t) of the transmission channel is expressed in the
following form:
H ( z ) = = 0 L - 1 h z - ##EQU00009##
[0090] In known manner, the time reversal of the transmission
channel has the transform H*(z.sup.-1) and its impulse response is
h*(-t). It is also defined by the complex coefficient vector
h.sup.RT=(h*.sub.L-1, h*.sub.L-2, . . . , h*.sub.0).
[0091] The estimated coefficient vector for the channels is written
as follows h=(h.sub.0, h.sub.1, . . . , h.sub.L-1). Assuming that
the channel estimate is ideal (i.e. that the channel coefficients
are accurately estimated, i.e. h=h, or in equivalent manner
h.sub.i=h.sub.i for i=0, . . . , L-1), the time-reversal prefilter
has the following z transform:
H ^ * ( z - 1 ) H * ( z - 1 ) = = 0 L - 1 h * z ##EQU00010##
[0092] The equivalent channel seen by the OFDM-OQAM modulation is
thus as follows:
C ( z ) = H ( z ) H * ( z - 1 ) = , k = 0 L - 1 h h k * z - ( - k )
##EQU00011##
which can also be written in the following form:
C ( z ) = = 0 L - 1 h 2 c 0 + k .noteq. ( h k h * z - ( k - ) + h k
* h z ( k - ) ) ##EQU00012##
or in frequency terms, writing z=exp(jq):
C ( exp ( j w ) ) = = 0 L - 1 h 2 c 0 + 2 k .noteq. ( ( h k h * )
cos ( k - ) w ) - ( h k * h ) sin ( ( k - ) w ) ) ##EQU00013##
[0093] The equivalent channel is thus a symmetrical conjugate
channel in which the central part c.sub.0 is a real coefficient
that follows a .chi..sup.2 distribution with 2L degrees of freedom
(the other coefficients having a normal distribution). From the
above equations, it can also be seen that because of the symmetry
of the coefficients of the equivalent channel, its transform C(z)
is a real function. It should be observed that this reasoning is
valid regardless of whether the channel is represented discretely
or continuously.
[0094] In other words, in accordance with the invention, the
symbols from the OFDM-OQAM modulator, which carry symbols with real
values, are subsequently transmitted over a real equivalent channel
C(z) obtained by convolution of the time-reversal prefilter and of
the transmission channel. As a result, the constraint of real
orthogonality on the carrier frequencies that is true for the
prototype filter of the OFDM-OQAM modulator remains true after
passing via this equivalent channel. In other words, no imaginary
interference term intrinsic to the OQAM modulation is created as a
result of propagation in the channel H(z). The invention thus
astutely combines the various orthogonality constraints associated
with distinct domains, namely with OFDM-OQAM modulation, time
reversal, and, if using MISO, with space-time coding or with
space-frequency coding.
[0095] With reference to FIGS. 2 to 5, there follows a description
of two particular implementations of the invention in which the
principle of time reversal is applied in accordance with the
invention to OFDM-OQAM modulation, both in the context of a SISO
system and in the context of a MISO system.
[0096] FIG. 2 shows a communications system 1 in accordance with
the invention, in a first embodiment.
[0097] The communications system 1 is a SISO system having a
transmit antenna TX and a receive antenna RX. In accordance with
the invention, it comprises: [0098] a transmitter device 1A in
accordance with the invention; and [0099] a receiver device 1B in
accordance with the invention.
[0100] The transmitter and receiver devices 1A and 1B are separated
by a transmission channel 2 that is assumed in this example to be
frequency selective and made up of L complex paths distributed in a
normal distribution. The complex coefficients of the transmission
channel 2 are written h=(h.sub.0, h.sub.1, . . . , h.sub.L-1).
[0101] In this example, the transmitter device 1A has the hardware
architecture of a computer. In particular, it comprises a control
unit 3 having a processor, a random access memory (RAM) 4, a ROM 5,
and communications means 6 including the transmit antenna TX. The
communications means 6 are controlled by the control unit 3 to
transmit digital signals (such as a multicarrier signal) over the
transmit antenna TX, and in this example they also include a
receive antenna (not shown) enabling the transmitter device 1A to
receive signals, e.g. signals coming from the receiver device 1B
and containing an estimate of the transmission channel between the
transmit antenna TX and the receive antenna RX.
[0102] The ROM 5 of the transmitter device 1A constitutes a storage
medium in accordance with the invention that is readable by the
processor of the control unit 3 and that stores a computer program
in accordance with the invention having instructions for executing
steps of a multicarrier signal transmission method in accordance
with the invention, as described below with reference to FIG. 3 in
a particular variant implementation.
[0103] In similar manner, the receiver device 1B in this example
has the hardware architecture of a computer. In particular, it
comprises a control unit having a processor, a RAM, a ROM, and
communications means including the receive antenna RX. The
communications means are controlled by the control unit to receive
digital signals (such as the multicarrier signal) via the receive
antenna RX, and in this example they also include a transmit
antenna enabling the receiver device 1B to send signals, e.g. such
as signals to the transmitter device 1A containing an estimate of
the transmission channel between the transmit antenna TX and the
receive antenna RX.
[0104] The ROM of the receiver device constitutes a storage medium
in accordance with the invention that is readable by the processor
of the control unit and that stores a computer program in
accordance with the invention including instructions for executing
steps of a multicarrier signal reception method in accordance with
the invention, as described below with reference to FIG. 3 in a
particular variant implementation.
[0105] FIG. 3 shows the main steps of the method of transmitting at
least one multicarrier signal, of a method of receiving a
multicarrier signal, and of a communications method of the
invention as performed respectively in the first embodiment by the
transmitter device 1A, by the receiver device 1B, and by the
communications system 1 of FIG. 2.
[0106] In this first embodiment, a sequence of bits is transformed
into a run of real-value symbols (a.sub.1, a.sub.2, . . . ) in a
phase and amplitude modulation (PAM) type constellation (step E10),
in known manner. The invention applies to any type of OQAM
modulation, and the PAM constellation may be of any order, and
indeed the constellation used may be a constellation other than a
PAM constellation, for example it may be a phase-shift keying (PSK)
constellation.
[0107] The bits of the sequence of bits may also be bits coded
using an error-correcting code such as a turbocode or a
convolutional code, and/or may have been subjected to time
interleaving prior to transformation into PAM symbols.
[0108] The real PAM symbols (also referred to as real value PAM
symbols) are then distributed over M subcarriers via a
series-to-parallel conversion step (step E20) so as to form
OFDM-OQAM symbols in the frequency domain. The real PAM symbol
associated with the mth subcarrier at instant nT/2 is written
a.sub.m,n, where T designates the duration of one complex
symbol.
[0109] The real symbols a.sub.m,n distributed over the M
subcarriers are then modulated using an OFDM-OQAM modulation scheme
(i.e. each real symbol a.sub.m,n modulates a subcarrier m of the
signal), in particular with the help of an inverse fast Fourier
transform IFFT and a prototype filter F (step E30) in known manner
that is not described in detail herein.
[0110] The discrete OFDM/OQAM multicarrier signal s[k] in baseband
can be written in the following form (after parallel-to-series
conversion in step E40):
s [ k ] = m = 0 M - 1 n = - .infin. + .infin. a m , n f [ k - n M 2
] j 2 .pi. M m ( k - ( LF - 1 ) 2 ) j .phi. m , n f m , n [ k ]
##EQU00014##
[0111] In the presently-described example
.phi. m , n = .pi. 2 ( m + n ) . ##EQU00015##
This phase serves to guarantee a quadrature rule between two
symbols that are adjacent not only in time but also in frequency
(i.e. on two adjacent carriers). In a variant, it is possible to
use other phase values that guarantee this rule, such as for
example
.phi. m , n = .pi. 2 ( m + n ) + .phi. 0 , ##EQU00016##
where .phi..sub.0 is arbitrary.
[0112] The prototype filter F is a filter of length LF satisfying a
real orthogonality constraint between the subcarriers, given
by:
{ ( f m , n , f m ' , n ' ) } = { k = - .infin. + .infin. f m , n [
k ] f m ' , n ' * [ k ] } = .delta. m , m ' .delta. n , n '
##EQU00017##
[0113] In other words <f.sub.m,n,f.sub.m',n'> is purely
imaginary for (m,n).noteq.(m',n').
[0114] Various known prototype filters satisfy such a condition and
can be used in the context of the invention, such as in particular
an IOTA prototype filter or the TFL1 filter as described in
document D1.
[0115] In accordance with the invention, the OFDM-OQAM symbols
forming the multicarrier signal s are then filtered by a
time-reversal prefilter TR (i.e. a matched filter) (step E50),
defined from an estimate of the transmission channel 2. More
precisely, the impulse response (or in equivalent manner the
coefficients) of the prefilter TR is obtained on the basis of the
estimate h (t) of the impulse response (or of the coefficients) of
the transmission channel 2 as reversed in time and conjugated.
[0116] In other words, the vector h.sup.TR of the coefficients of
the time-reversal prefilter TR are given by:
h.sup.TR=(h*.sub.L-1,h*.sub.L-2, . . . ,h*.sub.0)
where h=(h.sub.0, h.sub.1, . . . , h.sub.L-1) designates the
estimated coefficients of the transmission channel. It is possible
to normalize the coefficients of the time-reversal prefilter in
order to avoid modifying the transmission power of the multicarrier
signal.
[0117] The multicarrier signal resulting from this filtering step
is written x, where:
x=sh.sup.TR
where designates the convolution operator.
[0118] Thus, by definition of the prefilter TR, it is necessary to
have knowledge at the transmitter device 1A about the coefficients
of the propagation channel 2, in other words it is necessary to
have an estimate of the coefficients h=(h.sub.0, h.sub.1, . . . ,
h.sub.L-1) of the propagation channel 2.
[0119] Various channel estimation techniques may be used for
estimating the propagation channel 2, such as for example the
techniques of making an estimate by means of a preamble, as
described for OFDM/OQAM modulation in document D1.
[0120] In the presently-described example, it is assumed that this
estimate is made at the receiver device 1B and is then returned
over a return channel from the receiver device 1B to the
transmitter device 1A. This type of operation is conventionally
used in frequency-division duplex (FDD) systems operating in a
closed loop.
[0121] In a variant, for certain communications systems such as
systems using duplex transmission of the time-division duplex (TDD)
type, it is generally accepted that there is propagation channel
reciprocity between the up link and the down link. For such
systems, a channel estimate made by the transmitter on the basis of
a signal received from the receiver can then be used for
determining the coefficients of the time-reversal prefilter TR.
[0122] The filtered OFDM-OQAM symbols of the multicarrier signal x
are then transmitted over the transmission channel 2 via the
transmit antenna TX. The multicarrier signal x is a signal in
accordance with the invention.
[0123] The multicarrier signal received by the receive antenna RX
of the receiver device 1B and after propagation of the signal x in
the channel 2 is written y.
[0124] The received multicarrier signal y is then demodulated in
conventional manner as known to the person skilled in the art using
the following steps of the reception method of the invention as
performed by the receiver device 1B: [0125] series-to-parallel
conversion E60 (inverse of step E40); [0126] OFDM/OQAM demodulation
E70 using a Fourier transform and one equalizer ZF per carrier
having a single coefficient given by
.SIGMA..sub.l=0.sup.L-1.parallel.h.sub.l.parallel..sup.2 (naturally
it is possible to envisage using other equalizers, such as for
example an equalizer using a minimum mean square error (MMSE)
criterion); [0127] parallel-to-series conversion E80 of the
demodulated real value symbols a.sub.m,n, m=1, . . . , M on the M
subcarriers (inverse of step E20); and [0128] symbol-to-bit
conversion E90 in compliance with the constellation selected on
transmission and applied in step E10. Deinterleaving and decoding
of the bits may then be performed if the bits processed in step E10
were bits that had already been coded and interleaved, by using an
operation that is the inverse of that performed on
transmission.
[0129] As mentioned above, the transmission method of the invention
advantageously makes it possible to eliminate the imaginary
intrinsic interference term generated by using OFDM/OQAM
modulation.
[0130] By using calculations similar to those described in document
D1 as applied to the context of the invention to the equivalent
channel written HR obtained by convolution of h.sup.TR and h, and
assuming that the channel estimate available at the transmitter
device 1A is perfect, it can be shown that the demodulated signal
for the m.sup.th subcarrier at instant n may be written in the
following form:
y.sub.m,n=H.sub.m,n.sup.Ra.sub.m,n+jI.sub.m,n.eta..sub.m,n
where H.sub.m,n.sup.R designates the real coefficient of the
equivalent channel H.sup.R at the time-frequency position (m,n)
and:
I m , n = p , q , ( p , q ) .noteq. ( 0 , 0 ) a m + p , n + q H m +
p , n + q R f m + p , n - q m , n ##EQU00018##
I.sub.m,n is thus a real quantity (unlike the above-described
interference J.sub.m,n for OFDM-OQAM without time reversal), and it
can easily be eliminated by taking into consideration only the real
portion of the signal y.sub.m,n. Simple equalization of ZF type
with one coefficient per subcarrier then suffices to obtain an
estimate a.sub.m,n of the symbols a.sub.m,n transmitted by the
transmitter device 1A, i.e.:
a.sub.m,n=a.sub.m,n+(.eta..sub.m,n)
[0131] It can thus readily be understood that by using a
time-reversal prefilter of the propagation channel, the invention
provides a transmission scheme that does not suffer from loss of
spectrum efficiency while preserving a reception scheme that is
simple.
[0132] It is also possible to apply this same principle to a system
having a plurality of transmit antennas and a single receive
antenna (a MISO system).
[0133] A second embodiment is thus described below with reference
to FIGS. 4 and 5 in which the time-reversal principle is applied in
accordance with the invention to OFDM-OQAM modulation in the
context of a MISO system using a real orthogonal space-time code
with a coding rate of 1. The coding rate of a space-time (or
space-frequency) code applied to N transmit antennas is defined as
the ratio of the number of useful symbols over the number of symbol
durations needed to transmit them.
[0134] FIG. 4 shows a MISO communications system 1' in accordance
with the invention in a second embodiment.
[0135] In accordance with the invention, the communications system
1' comprises: [0136] a transmitter device 1A' in accordance with
the invention and having N transmit antennas TX1, TX2, . . . , TXN;
and [0137] a receiver device 1B' in accordance with the invention
and fitted with a single receive antenna RX.
[0138] The transmitter and receiver devices 1A' and 1B' are
separated by a transmission channel 2' made up of N subchannels (or
channels) 2_1', 2_2', 2_N'. More precisely, in the context of a
MISO system, each transmit antenna TXj, j=1, . . . , N of the
transmitter device 1A' is separated from the receive antenna RX of
the receiver device 1B' by a transmission channel 2_1', 2_2', . . .
, 2_N, each channel 2.sub.--n' being assumed in this example to be
frequency selective and made up of respectively of Lj, j=1, . . . ,
N complex paths distributed in a normal distribution. The complex
coefficients of the propagation channel 2.sub.--j', j=1, . . . , N
are written:
h.sup.(j)=(h.sub.0.sup.(j),h.sub.1.sup.(j), . . .
,h.sub.Lj-1.sup.(j))
[0139] For simplification purposes, it is also assumed that L1=L2=
. . . =LN=L. Nevertheless, the invention is also applicable for
lengths Lj, j=1, . . . , N that are different.
[0140] In this example, the transmitter device 1A' has the hardware
architecture of a computer. In particular, it comprises a control
unit 3' with a processor, a RAM 4', a ROM 5', and communications
means 6' including the transmit antennas TX1, TX2, . . . , TXN. The
communications means 6' are controlled by the control unit 3' to
transmit digital signals (such as multicarrier signals) via the
transmit antennas TX1, TX2, . . . , TXN, and in this example they
also include at least one receive antenna (not shown) enabling the
transmitter device 1A' to receive signals, such as for example
signals from the receiver device 1B' and containing an estimate of
the transmission channels between the antennas TX1, TX2, . . . ,
TXN and the antenna RX.
[0141] The ROM 5' of the transmitter device 1A' constitutes a data
medium in accordance with the invention that is readable by the
processor of the control unit 3' and that stores a computer program
in accordance with the invention, including instructions for
executing steps of a multicarrier signal transmission method in
accordance with the invention in its second implementation, as
described below with reference to FIG. 5, in a particular variant
implementation.
[0142] The receiver device 1B' has hardware architecture similar to
the receiver device 1B described above for the first embodiment. In
the presently-described implementation, the instructions for
executing steps of the reception method in accordance with the
invention in the second implementation are contained in a computer
program stored in a ROM of the receiver device 1B'.
[0143] FIG. 5 shows the main steps of a method of transmitting a
multicarrier signal, of a reception method, and of a communications
method in accordance with the invention as performed respectively
by the device 1A', the device 1B', and the communications system 1'
of FIG. 4 in a second implementation in which use is made on
transmission of a real orthogonal space-time code of coding rate 1
for distributing the OFDM-OQAM symbols over the N transmit
antennas.
[0144] More precisely, in this second implementation, a sequence of
bits (which may possibly be coded and interleaved as in the first
implementation) is transformed into a run of real symbols (a.sub.1,
a.sub.2, . . . ) of a PAM type constellation (step F10') in known
manner. Since the invention applies to any type of OQAM modulation,
the PAM constellation may be of any order and it is possible to
envisage using other constellations.
[0145] The real PAM symbols (also referred to as real value PAM
symbols) are then distributed over M subcarriers and N antennas
during a series-to-parallel conversion step (step F20) so as to
form OFDM-OQAM symbols in the frequency domain for each transmit
antenna. The real PAM symbol associated with the m.sup.th
subcarrier at instant n is written a.sub.m,n.
[0146] The real PAM symbols a.sub.m,n are then coded during a
coding step, in this example using one space dimension and one time
dimension, so as to generate N coded symbol sequences for the N
transmit antennas (step F30).
[0147] More precisely, as mentioned above, in the
presently-described second implementation, it is envisaged coding
symbols a.sub.m,n over N transmit antennas by means of a real
orthogonal space-time code of coding rate 1 (i.e. all of the
components of the coding matrix GRN defining this code are real).
Such codes are known to the person skilled in the art, and they are
described in particular in the above-mentioned document by S.
Alamouti for a number of transmission antennas N equal to 2, and in
the document by V. Tarokh et al. entitled "Space-time block codes
from orthogonal designs", IEEE Transactions on Information Theory,
Vol. 45, 1999, for an arbitrary number of transmit antennas.
[0148] In general, these codes are defined by a coding matrix of
dimensions N.times.N. To illustrate the invention, it is assumed in
this example that N=2, and it is envisaged to perform space-time
coding of real symbols a.sub.m,n distributed over the M subcarriers
as defined by the following orthogonal real coding matrix GR2
deduced from Alamouti's matrix GC2 described above:
GR 2 = [ a m , n - a m , n + 1 a m , n + 1 a m , n ] antenna
.dwnarw. , time .fwdarw. ##EQU00019##
the j.sup.th row of the matrix GR2 giving the symbols transmitted
during each symbol time over antenna TXj.
[0149] For N=4, an example of a real orthogonal coding matrix GR4
of coding rate 1 that is suitable for application during step F30
in the context of the invention is as follows:
GR 4 = [ a m , n a m , n + 1 a m , n + 2 a m , n + 3 - a m , n + 1
a m , n - a m , n + 3 a m , n + 2 - a m , n + 2 a m , n + 3 a m , n
- a m , n + 1 - a m , n + 3 - a m , n + 2 a m , n + 1 a m , n ]
antenna .dwnarw. , time .fwdarw. ##EQU00020##
[0150] Naturally, the invention applies to other real orthogonal
space-time codes of coding rate 1.
[0151] Thus, during coding step F30, for each subcarrier m, m=1, .
. . , M, the matrix GR2 is applied to the real value symbols
a.sub.m,n associated with that subcarrier. This produces N coded
symbol sequences for the N antennas, i.e. each resulting coded
sequence is associated with a respective transmit antenna. These
coded symbols are real symbols since the matrix GR2 is a real
matrix.
[0152] The N coded symbol sequences are then modulated respectively
for the N transmit antennas using the OFDM-OQAM modulation scheme
(i.e. each coded symbol of a sequence associated with a transmit
antenna modulates a subcarrier with an OFDM-OQAM symbol associated
with the antenna) (step F41 for the antenna TX1, F42 for the
antenna TX2, etc. grouped together as step F40), in particular with
the help of an inverse fast Fourier transform IFFT and a prototype
filter f (step F40), and then converted in a parallel-to-series
conversion operation performed independently for each antenna (step
F50 grouping together steps F51, F52, F5N performed respectively
for each of the antennas TX1, TX2, . . . , TXN) in similar manner
to the first implementation for antenna TX.
[0153] Then, in accordance with the invention, for each antenna
TXj, j=1, . . . , N, the OFDM-OQAM symbols forming the coded
OFDM/OQAM multicarrier signal s(j) for this antenna TXj are
filtered by a time-reversal prefilter TRj (step F61 for the antenna
TX1, F62 for the antenna TX2, . . . , F6N for the antenna TXN,
which steps are grouped together as filter step F60), defined on
the basis of an estimate of the transmission channel
2.sub.--j'.
[0154] The impulse response of the prefilter TRj applied to antenna
TXj, j=1, . . . , N is obtained from the time reverse and
conjugated impulse response of an estimate of the transmission
channel 2.sub.--j' between the transmit antenna TXj and the receive
antenna RX. In other words, the vector h.sup.TR(j) of the
coefficients of the time-reversal prefilter TRj for transmit
antenna TXj are given by:
h.sup.TR(j)=((h.sub.L-1.sup.(j))*,(h.sub.L-2.sup.(j))*, . . .
,(h.sub.0.sup.(j))*)
where h.sup.(j)=(h.sub.0.sup.(j),h.sub.1.sup.(j), . . . ,
h.sub.L-1.sup.(j)) designates the estimated coefficients of the
propagation channel 2.sub.--j' between the transmit antenna TXj and
the receive antenna RX.
[0155] The coefficients of the filters TRj may be normalized over
the set of transmit antennas in order to avoid modifying the
transmission power of the multicarrier signals. The resulting
multicarrier signal for each antenna TXj, j=1, . . . , N after
these filter steps is written x.sup.(j) with
x.sup.(j)=s.sup.(j)h.sup.TR(j)
[0156] The estimate h.sup.(j)=(h.sub.0.sup.(j), h.sub.1.sup.(j), .
. . , h.sub.L-1.sup.(j)) of the coefficients of each channel
2.sub.--j' may be obtained as in the first implementation, either
via a return path from the receiver 1B' or by assuming that the
reciprocity for the propagation channel on the basis of the
coefficients estimated during transmission in the direction 1B' to
1A'.
[0157] The filtered multicarrier signals x.sup.(j), j=1, . . . , N
are then transmitted over the propagation channel 2' via their
respective transmit antennas TXj, j=1, . . . , N. The signal made
up of the multicarrier signals x.sup.(j) is a signal in accordance
with the invention.
[0158] The multicarrier signal received via the receive antenna RX
of the receiver device 1B' after the signals x.sup.(j), j=1, . . .
, N have propagated over the channel 2' is written y'. The received
signal y' is equal to the sum of the signals received from each
transmit antenna TXj, j=1, . . . , N, i.e.:
y ' = j = 1 N x ( j ) h ( j ) + .eta. ##EQU00021##
where .eta. designates Gaussian additive white noise.
[0159] The various symbols of the received multicarrier signal y'
are distributed during series-to-parallel conversion over M
subcarriers (step F70), and then a fast Fourier transform FFT is
applied in order to demodulate the OFDM-OQAM symbols (step F80) in
similar manner to the first implementation.
[0160] In the example of space-time coding performed with the help
of the above-described matrix GR2, for each carrier m, the symbols
obtained after FFT transformation for each carrier m at instants nT
and (n+1)T are given by the following equations:
y'.sub.m,n=H.sub.m,n.sup.(R)0a.sub.m,n+H.sub.m,n.sup.(R)1a.sub.m,n+1+.et-
a..sub.m,n
y'.sub.m,n+1=-H.sub.m,n.sup.(R)0a.sub.m,n+1+H.sub.m,n.sup.(R)1a.sub.m,n+-
.eta..sub.m,n+1
where .eta..sub.m,n and .eta..sub.m,n+1 designate the components of
the Gaussian additive white noise .eta. for the m.sup.th carrier at
instant nT.
[0161] This expression may also be written in matrix form as
follows:
[ y m , n ' y m , n + 1 ' ] = [ H m , n R ( 0 ) H m , n R ( 1 ) H m
, n R ( 1 ) - H m , n R ( 0 ) ] HC [ a m , n a m , n + 1 ] + [
.eta. m , n .eta. m , n + 1 ] ##EQU00022##
[0162] The matrix HC is an orthogonal matrix so it is possible
using simple linear processing that is itself known to retrieve the
transmitted symbols a.sub.m,n. An estimate a.sub.m,n of the real
symbols is thus obtained on the basis of the symbols y'.sub.m,n
from the FFT by using this linear processing performed on each
subcarrier m (step F90).
[0163] After parallel-to-series conversion of the real symbols as
estimated for each carrier (step F100, performing processing that
is the inverse of step F20), the symbols are converted into bits
(step F110) using the constellation selected on transmission as
applied in step F10. The bits are then deinterleaved and decoded if
the bits that were processed in step F10 were themselves coded and
interleaved, by using an operation that is the inverse of the
operation that was performed on transmission.
[0164] Thus, as mentioned above, the transmission method of the
invention advantageously makes it possible to eliminate the
intrinsic interference terms generated by using OFDM/OQAM
modulation, including in a MISO system.
[0165] It can thus be understood that by applying a propagation
channel time-reversal prefilter for each transmit antenna, the
invention provides a transmission scheme that does not suffer from
loss of spectrum efficiency and that makes it possible to use
space-time codes with a coding rate of 1, while preserving a
reception scheme that is simple.
[0166] In the second implementation described herein, orthogonal
real space-time coding is described with a coding rate of 1 for
symbols a.sub.m,n (i.e. the symbols a.sub.m,n are distributed in
time and in space). Nevertheless, the invention also applies to
orthogonal coding with a coding rate of 1 of real value symbols
a.sub.m,n using a space dimension and a frequency dimension
(space-frequency coding). For N=2 transmit antennas, such a code
may be defined by way of example by the following orthogonal real
matrix GRF2:
GRF 2 = [ a m , n - a m + 1 , n a m + 1 , n a m , n ] antenna
.dwnarw. , carrier .fwdarw. ##EQU00023##
the j.sup.th row of the matrix GRF2 giving the symbols transmitted
on each carrier via the antenna TXj.
[0167] In addition, in the second implementation described herein,
an orthogonal real code of coding rate 1 is described.
Nevertheless, the invention also applies to non-orthogonal
space-time real codes (in which case non-linear decoding should be
performed on reception), and also to codes having a coding rate of
less than 1.
[0168] In the second implementation described herein, the use of a
space-time code on transmission is described. Nevertheless, the
invention also applies to a "simple" MISO system that does not
perform space-time or space-frequency coding (and that therefore
does not require a linear processing module on reception).
[0169] In the implementations described herein, OFDM-OQAM type
multicarrier signal transmission is described. Nevertheless, the
principle of time reversal applied to OQAM modulation can also be
used for transmitting single-carrier signals. It should be observed
that a single-carrier signal is a special case of a multicarrier
signal in which M=1 (in other words, the reasoning set out in the
present description for a multicarrier situation can still be
applied, by putting M=1, where M is the number of carriers).
[0170] Under such circumstances, for each transmit antenna of the
transmitter device, the transmission method comprises: [0171] a
step of filtering OQAM symbols associated with that transmit
antenna by means of a time-reversal filter defined from an estimate
of a transmission channel between the transmit antenna and the
receive antenna; and [0172] a step of transmitting the filtered
OQAM symbols via the transmit antenna over the channel.
[0173] It is also possible to envisage that the method of
transmitting at least one single-carrier signal presents in
combination some or all of the above-mentioned characteristics
(e.g. real and orthogonal space-time or space-frequency coding,
etc.).
[0174] In general, the time-reversal principle described above may
be applied to any signal making use of real orthogonality in order
to obtain advantages that are similar to those of the
invention.
* * * * *